U.S. patent number 3,993,132 [Application Number 05/588,124] was granted by the patent office on 1976-11-23 for thermal recovery of hydrocarbons from tar sands.
This patent grant is currently assigned to Texaco Exploration Canada Ltd.. Invention is credited to Phillip J. Cram, David A. Redford.
United States Patent |
3,993,132 |
Cram , et al. |
November 23, 1976 |
Thermal recovery of hydrocarbons from tar sands
Abstract
A method for the recovery of low API gravity viscous oils or
bitumen from a subterranean formation by the injection of a mixture
of an oxygen-containing gas and steam having an optimum gas/steam
ratio that is decreased as the cumulative amount of steam is
increased.
Inventors: |
Cram; Phillip J. (Calgary,
CA), Redford; David A. (Fort Saskatchewan,
CA) |
Assignee: |
Texaco Exploration Canada Ltd.
(CA)
|
Family
ID: |
24352575 |
Appl.
No.: |
05/588,124 |
Filed: |
June 18, 1975 |
Current U.S.
Class: |
166/261;
166/401 |
Current CPC
Class: |
E21B
43/164 (20130101); E21B 43/243 (20130101) |
Current International
Class: |
E21B
43/243 (20060101); E21B 43/16 (20060101); E21B
043/24 () |
Field of
Search: |
;166/272,261,256 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Novosad; Stephen J.
Assistant Examiner: Suckfield; George A.
Attorney, Agent or Firm: Whaley; Thomas H. Ries; Carl G.
Bauer; Charles L.
Claims
We claim:
1. In a method for the recovery of viscous hydrocarbons from a
subterranean hydrocarbon-bearing formation, traversed by at least
one injection well and at least one production well, and having
fluid communication therebetween, wherein a mixture of an
oxygen-containing gas and steam is injected via said injection well
and hydrocarbons are produced via said production well, said
mixture being injected at a temperature corresponding to the
saturation temperature for saturated steam at the pressure of said
formation whereby a low temperature oxidation is established in
said formation, the improvement comprising decreasing the relative
concentration of oxygen-containing gas in said mixture during the
injection of said mixture.
2. The method of claim 1 wherein said oxygen-containing gas is
substantially pure oxygen.
3. The method of claim 1 wherein the oxygen-containing gas is
air.
4. The method of claim 1 wherein the oxygen-containing gas
comprises oxygen, nitrogen, carbon dioxide, flue gas and mixtures
thereof.
5. The method of claim 1 wherein said formation is first
repressured to a pressure corresponding to a temperature of
saturated steam in the range of 250.degree. F to 500.degree. F.
6. A method of recovery of viscous hydrocarbons from a subterranean
hydrocarbon-bearing formation traversed by at least one injection
well and at least one production well and having fluid
communication therebetween comprising the steps of;
a. injecting into said formation via said injection well a mixture
comprising an oxygen-containing gas and steam said mixture being
injected at a temperature corresponding to the saturation
temperature for saturated steam at the pressure of the formation
whereby a low temperature oxidation is established in said
formation wherein the ratio of the oxygen-containing gas to steam
in said mixture is progressively decreased during the injection of
said mixture,
b. producing said hydrocarbons via said production well.
7. The method of claim 6 wherein after about one pore volume of
steam is injected, said ratio of gas to steam is progressively
decreased from a range of from about 0.5 to about 0.7 MSCF/bbl to a
range of about 0.2 to 0.4 MSCF/bbl.
8. The method of claim 6 wherein said oxygen-containing gas is
substantially pure oxygen.
9. The method of claim 6 wherein the oxygen-containing gas is
air.
10. The method of claim 6 wherein the oxygen-containing gas
comprises oxygen, nitrogen, carbon dioxide, flue gas and mixtures
thereof.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an improved method for the
recovery of oil from subterranean hydrocarbon-bearing formations
containing low API gravity viscous oils or bitumen. More
particularly, the invention relates to the production of bitumen
and hydrocarbons from reservoirs of low mobility, such as tar sand
formations.
The recovery of viscous oils from formations and bitumen from tar
sands has generally been difficult. Although some improvement has
been realized in stimulating recovery of heavy oils, i.e., oils
having an API gravity in the range of 10.degree. to 25.degree. API,
little, if any, success has been realized in recovering bitumen
from tar sands. Bitumen can be regarded as highly viscous oils
having a gravity in the range of about 5.degree. to 10.degree. API
and contained in an essentially unconsolidated sand referred to as
tar sands.
Vast quantities of tar sands are known to exist in the Athabasca
region of Alberta, Canada. While these deposits are estimated to
contain several hundred billion barrels of oil or bitumen, recovery
therefrom using conventional in-situ techniques has not been too
successful. The reasons for the lack of success relate principally
to the fact that the bitumen is extremely viscous at the
temperature of the formation, with consequent low mobility. The
viscosity of the tar sands from the Athabasca deposits, for
example, is in the range of several million centipoise at the
average formation temperature of about 40.degree. F, so that the
bitumen is essentially immobile at formation temperature. In
addition, these tar sand formations have very low permeability,
despite the fact they are unconsolidated.
Since it is known that the viscosity of oil decreases markedly with
an increase in temperature, thereby improving its mobility, thermal
recovery techniques have been investigated for recovery of bitumen
from tar sands. These thermal recovery methods generally include
steam injection, hot water injection and in-situ combustion.
Typically, such thermal techniques employ an injection well and a
production well traversing the oil-bearing or tar sand formation.
In a steam operation employing two wells, steam is introduced into
the formation through the injection well. Upon entering the
formation, the heat transferred by the hot fluid functions to lower
the viscosity of oil, thereby improving its mobility, while the
flow of the hot fluid functions to drive the oil toward the
production well from which it is produced.
In the conventional forward in-situ combustion operation, an
oxygen-containing gas, such as air, is introduced into the
formation via a well, and combustion of the in-place crude adjacent
the wellbore is initiated by one of many known means, such as the
use of a downhole gas-fired heater or downhole electric heater or
chemical means. Thereafter, the injection of the oxygen-containing
gas is continued so as to maintain a combustion front which is
formed and to drive the front through the formation toward the
production well.
As the combustion front advances through the formation, a swept
area consisting, ideally, of a clean sand matrix is created behind
the front. Ahead of the advancing front various contiguous zones
are built up that also are displaced ahead of the combustion front.
These zones may be envisioned as a distillation and cracking zone,
a condensation and vaporization zone, an oil bank and a virgin or
unaltered zone.
The temperature of the combustion front is generally in the range
of 750.degree.-1100.degree. F. The heat generated in this zone is
transferred to the distillation and cracking zone ahead of the
combustion front where the crude undergoes distillation and
cracking. In this zone a sharp thermal gradient exists wherein the
temperature drops from the temperature of the combustion front to
about 300.degree.-450.degree. F. As the front progresses and the
temperature in the formation rises, the heavier molecular weight
hydrocarbons of the oil become carbonized. These coke-like
materials are deposited on the matrix and are the potential fuel to
sustain the progressive in-situ combustion.
Ahead of the distillation and cracking zone is a condensation and
vaporization zone. This zone is a thermal plateau and its
temperature is in the range of from about 200.degree. F to about
450.degree. F, depending upon the pressure and the distillation
characteristics of the fluids therein. These fluids consist of
water and steam and hydrocarbon components of the crude.
Ahead of the condensation and vaporization zone is an oil bank
which forms as the in-situ combustion progresses and the formation
crude is displaced toward the production well. This zone of high
oil saturation contains not only reservoir fluids, but also
condensate, cracked hydrocarbons and gaseous products of combustion
which eventually reach the production well from which they are
produced.
Various improvements relating to in-situ combustion are described
in the prior art that relate to the injection of water, either
simultaneously or intermittently with the oxygen-containing gas, to
scavenge the residual heat in the formation behind the combustion
front, thereby increasing recovery of oil. Prior art also discloses
regulating the amount of water injected so as to improve
conformance or sweep efficiency.
Experience has generally shown that these conventional thermal
techniques have not been altogether successful when applied to the
recovery of heavy oils or bitumen. Where the hydrocarbons sought to
be produced have a low API gravity, the build-up of the oil bank
ahead of the thermal front occurs to a great extent. Since the heat
transfer is low ahead of the front, these heavy hydrocarbons become
cool and hence immobile, thereby causing plugging of the formation
with the result that the injection of either air in the case of
in-situ combustion, or steam in the case of steam, is no longer
possible.
The problems recited above become compounded when these techniques
are applied to the tar sands not only because of the very low API
gravity and very high viscosity of the bitumen, but also because of
the very low permeability of the tar sand formations.
Accordingly, it is an object of the present invention to provide an
improved thermal recovery method whereby both highly viscous, low
gravity crudes and bitumen can be recovered more efficiently. The
instant invention accomplishes this recovery of heavy oils and
bitumen by utilizing thermal methods wherein a mixture of an
oxygen-containing gas and steam is injected having an optimum ratio
of gas-to-steam that is controlled in relation to the cumulative
amount of steam injected.
SUMMARY OF THE INVENTION
This invention relates to an improved method of recovering low API
gravity viscous oils, and more particularly to the production of
bitumen from tar sands, by the injection of a mixture of an
oxygen-containing gas and steam having an optimum ratio of gas to
steam that is decreased as the cumulative amount of steam is
increased.
BRIEF DESCRIPTION OF THE FIGURE
The FIGURE shows the relationship between the ratio of the
cumulative barrels of steam injected/barrels of oil produced and
cumulative air-steam ratio.
DESCRIPTION OF THE PREFERRED EMBODIMENT
We have found that improved recovery of viscous or low API gravity
petroleum and bitumen from tar sands, can be obtained by injecting
a mixture of an oxygen-containing gas and steam, wherein the ratio
of the oxygen-containing gas to steam has an optimum value with
respect to the cumulative amount of steam injected.
In the copending and coassigned application, Ser. No. 481,581 filed
June 21, 1974, there is described a method for the recovery of
heavy oils or bitumens by the injection of a mixture of an
oxygen-containing gas and steam at a temperature corresponding to
the saturation temperature of steam for the pressure of the
formation, whereby low temperature oxidation is established and
maintained in-situ in a temperature range of
250.degree.-500.degree. F to enhance the recovery of petroleum.
We have now determined that enhanced recovery utilizing the use of
a mixture of an oxygen-containing gas and steam can be improved by
controlling the ratio of the oxygen-containing gas to the steam
injected. Furthermore, this ratio has an optimum value depending
upon the cumulative volumes of fluid injected.
To illustrate this invention, a series of laboratory tests were
performed using a tar sand from the McMurray formation in Alberta,
Canada. Approximately 190 pounds of tar sand were packed in a cell
approximately 15 inches long and 18 inches in diameter. The cell
was equipped for operating at controlled temperatures up to
420.degree. F and pressures of 500 psi, and contained simulated
suitable injection and production wells. In addition, the cell
contained many thermocouples so that both temperatures throughout
the cell could be measured and heat transfer rates could be
calculated.
The general procedure employed in the test involved the injection
of steam to establish fluid communication and to initiate
production, after which a mixture of an oxygen-containing gas such
as air and steam was injected. The injection pressure was about 300
psi and the temperature was about 417.degree. F, corresponding to
the saturation pressure of steam. The accompanying table gives the
conditions of the runs and the results.
______________________________________ Fluids Time Air/Steam
Recovery Run Injected (hr.) Ratio % (MSCF air/ bbl steam) 1 Steam
30 -- 31 2 Steam and Air 24 0.19 62 3 Steam and Air 24 0.35 35 4
Steam and Air 27 0.35 47 5 Steam and Air 25 0.35 47 6 Steam and Air
24 0.70 39 7 Steam and Air 24 1.40 41
______________________________________
The results are also plotted in the accompanying FIGURE in terms of
cumulative barrels of steam injected per barrels of oil produced
(bbl/bbl) against the cumulative air-steam ratio (MSCF/bbl), for
different cumulative pore volumes of steam injected.
The table shows the recoveries obtained by varying the ratio of the
air to the steam injected. The ratio is expressed in terms of
thousand cubic feet of air per barrel of steam injected. In the
accompanying FIGURE the results are plotted whereby the essence of
the invention is demonstrated. The cumulative barrels of steam
injected per barrel of oil produced is plotted against the air to
steam ratio (MSCF/bbl) for given cumulative pore volumes of steam
injected. By pore volumes of steam injected is meant pore volumes
of water at 60.degree. F converted to steam at the injected
temperature and pressure.
The FIGURE shows that for a given cumulative pore volume of
injected steam there is a minimum value for the ratio of cumulative
steam injected to oil produced that occurs at a specific
air-to-steam ratio. The FIGURE also shows that there is a
beneficial effect from the simultaneous injection of air with the
steam as evidenced by the lower values of the ratio of the steam
injected to oil produced.
Thus, for the most efficient recovery of oil expressed in terms of
minimum steam injected per oil produced, there is disclosed a
process whereby the air-to-steam ratio is controlled in relation to
the pore volumes of cumulative injected steam. Generally as the
injection of steam is continued, the air-to-steam ratio should be
decreased.
In a broad aspect of the invention a hydrocarbon-bearing formation
containing a heavy crude or a tar sand containing bitumen is first
traversed by at least one injection well and one production well
and fluid communication is established such as by the injection of
air or nitrogen. It may be necessary to fracture the formation
and/or inject a solvent to obtain adequate transmissibility.
Thereafter, a mixture of the oxygen-containing gas and steam is
injected, the ratio of the constituents of the mixture being
adjusted as the injection of steam, in terms of cumulative pore
volumes, is continued.
For example where a mixture of air and steam is injected, at a
cumulative pore volume of injected steam of 1, the cumulative
air-to-steam ratio should be in the range of about 0.61. As the
cumulative pore volume of injected steam increases to 2, the said
ratio should be decreased to approximately 0.42, and as the pore
volume increases to 4, the said ratio should be decreased to
approximately 0.22.
While the temperature of the mixture is preferred to be in the
range of 250.degree. to 500.degree. F, this may be realized by
repressuring the formation to a pressure corresponding to that
temperature of saturated steam in the desired temperature range.
For example, the formation may first be repressured to about 300
psi so that the temperature of injected steam and oxygen-containing
gas can be in the range of 420.degree. F.
The oxygen-containing gas may be air, or a mixture of oxygen and
non-condensible gases such as nitrogen, carbon dioxide or flue gas,
or it may be substantially pure oxygen.
In summary, in accordance with this invention, more efficient
recovery of heavy oils or bitumen is accomplished by the injection
of a mixture of an oxygen-containing gas and steam wherein the
ratio of the oxygen-containing gas to steam is controlled and
regulated as the cumulative pore volumes of injected steam
increase. As the injection of steam continues, the ratio of the gas
to steam or the relative concentration of the gas in the injected
mixture is decreased, thereby minimizing the cumulative barrels of
steam injected per barrel of oil produced.
While the actual air-to-steam ratios may be different for given
reservoir conditions of temperature and pressure, and formation
characteristics, the desired air-to-steam ratios utilized for pore
volumes of steam injected may be determined or estimated from tests
performed as described herein for conditions simulating given
formations in which the disclosed method may be applied.
In one example after about one pore volume of steam has been
injected the ratio of the gas-to-steam in the injected mixture is
progressively decreased to a range of from about 0.5-0.7 MSCF/bbl
to a range of about 0.2-0.4 MSCF/bbl, when approximately 4 pore
volumes of steam have been injected.
* * * * *